Fixed #30446 -- Resolved Value.output_field for stdlib types.

This required implementing a limited form of dynamic dispatch to combine
expressions with numerical output. Refs #26355 should eventually provide
a better interface for that.

django/contrib/gis/db/models/functions.py

@@ -101,10 +101,13 @@ class SQLiteDecimalToFloatMixin:
     is not acceptable by the GIS functions expecting numeric values.
     """
     def as_sqlite(self, compiler, connection, **extra_context):
-        for expr in self.get_source_expressions():
+        copy = self.copy()
-            if hasattr(expr, 'value') and isinstance(expr.value, Decimal):
+        copy.set_source_expressions([
-                expr.value = float(expr.value)
+            Value(float(expr.value)) if hasattr(expr, 'value') and isinstance(expr.value, Decimal)
-        return super().as_sql(compiler, connection, **extra_context)
+            else expr
+            for expr in copy.get_source_expressions()
+        ])
+        return copy.as_sql(compiler, connection, **extra_context)
 
 
 class OracleToleranceMixin:

django/contrib/postgres/fields/ranges.py

@@ -173,8 +173,7 @@ class DateTimeRangeContains(PostgresOperatorLookup):
     def process_rhs(self, compiler, connection):
         # Transform rhs value for db lookup.
         if isinstance(self.rhs, datetime.date):
-            output_field = models.DateTimeField() if isinstance(self.rhs, datetime.datetime) else models.DateField()
+            value = models.Value(self.rhs)
-            value = models.Value(self.rhs, output_field=output_field)
             self.rhs = value.resolve_expression(compiler.query)
         return super().process_rhs(compiler, connection)
 

django/db/models/expressions.py

@@ -1,7 +1,9 @@
 import copy
 import datetime
+import functools
 import inspect
 from decimal import Decimal
+from uuid import UUID
 
 from django.core.exceptions import EmptyResultSet, FieldError
 from django.db import NotSupportedError, connection
@@ -56,12 +58,7 @@ class Combinable:
     def _combine(self, other, connector, reversed):
         if not hasattr(other, 'resolve_expression'):
             # everything must be resolvable to an expression
-            output_field = (
+            other = Value(other)
-                fields.DurationField()
-                if isinstance(other, datetime.timedelta) else
-                None
-            )
-            other = Value(other, output_field=output_field)
 
         if reversed:
             return CombinedExpression(other, connector, self)
@@ -422,6 +419,25 @@ class Expression(BaseExpression, Combinable):
     pass
 
 
+_connector_combinators = {
+    connector: [
+        (fields.IntegerField, fields.DecimalField, fields.DecimalField),
+        (fields.DecimalField, fields.IntegerField, fields.DecimalField),
+        (fields.IntegerField, fields.FloatField, fields.FloatField),
+        (fields.FloatField, fields.IntegerField, fields.FloatField),
+    ]
+    for connector in (Combinable.ADD, Combinable.SUB, Combinable.MUL, Combinable.DIV)
+}
+
+
+@functools.lru_cache(maxsize=128)
+def _resolve_combined_type(connector, lhs_type, rhs_type):
+    combinators = _connector_combinators.get(connector, ())
+    for combinator_lhs_type, combinator_rhs_type, combined_type in combinators:
+        if issubclass(lhs_type, combinator_lhs_type) and issubclass(rhs_type, combinator_rhs_type):
+            return combined_type
+
+
 class CombinedExpression(SQLiteNumericMixin, Expression):
 
     def __init__(self, lhs, connector, rhs, output_field=None):
@@ -442,6 +458,19 @@ class CombinedExpression(SQLiteNumericMixin, Expression):
     def set_source_expressions(self, exprs):
         self.lhs, self.rhs = exprs
 
+    def _resolve_output_field(self):
+        try:
+            return super()._resolve_output_field()
+        except FieldError:
+            combined_type = _resolve_combined_type(
+                self.connector,
+                type(self.lhs.output_field),
+                type(self.rhs.output_field),
+            )
+            if combined_type is None:
+                raise
+            return combined_type()
+
     def as_sql(self, compiler, connection):
         expressions = []
         expression_params = []
@@ -721,6 +750,30 @@ class Value(Expression):
     def get_group_by_cols(self, alias=None):
         return []
 
+    def _resolve_output_field(self):
+        if isinstance(self.value, str):
+            return fields.CharField()
+        if isinstance(self.value, bool):
+            return fields.BooleanField()
+        if isinstance(self.value, int):
+            return fields.IntegerField()
+        if isinstance(self.value, float):
+            return fields.FloatField()
+        if isinstance(self.value, datetime.datetime):
+            return fields.DateTimeField()
+        if isinstance(self.value, datetime.date):
+            return fields.DateField()
+        if isinstance(self.value, datetime.time):
+            return fields.TimeField()
+        if isinstance(self.value, datetime.timedelta):
+            return fields.DurationField()
+        if isinstance(self.value, Decimal):
+            return fields.DecimalField()
+        if isinstance(self.value, bytes):
+            return fields.BinaryField()
+        if isinstance(self.value, UUID):
+            return fields.UUIDField()
+
 
 class RawSQL(Expression):
     def __init__(self, sql, params, output_field=None):
@@ -1177,7 +1230,6 @@ class OrderBy(BaseExpression):
             copy.expression = Case(
                 When(self.expression, then=True),
                 default=False,
-                output_field=fields.BooleanField(),
             )
             return copy.as_sql(compiler, connection)
         return self.as_sql(compiler, connection)

django/db/models/lookups.py

@@ -6,7 +6,7 @@ from copy import copy
 from django.core.exceptions import EmptyResultSet
 from django.db.models.expressions import Case, Exists, Func, Value, When
 from django.db.models.fields import (
-    BooleanField, CharField, DateTimeField, Field, IntegerField, UUIDField,
+    CharField, DateTimeField, Field, IntegerField, UUIDField,
 )
 from django.db.models.query_utils import RegisterLookupMixin
 from django.utils.datastructures import OrderedSet
@@ -123,7 +123,7 @@ class Lookup:
         exprs = []
         for expr in (self.lhs, self.rhs):
             if isinstance(expr, Exists):
-                expr = Case(When(expr, then=True), default=False, output_field=BooleanField())
+                expr = Case(When(expr, then=True), default=False)
                 wrapped = True
             exprs.append(expr)
         lookup = type(self)(*exprs) if wrapped else self

docs/ref/models/expressions.txt

@@ -484,7 +484,15 @@ The ``output_field`` argument should be a model field instance, like
 after it's retrieved from the database. Usually no arguments are needed when
 instantiating the model field as any arguments relating to data validation
 (``max_length``, ``max_digits``, etc.) will not be enforced on the expression's
-output value.
+output value. If no ``output_field`` is specified it will be tentatively
+inferred from the :py:class:`type` of the provided ``value``, if possible. For
+example, passing an instance of :py:class:`datetime.datetime` as ``value``
+would default ``output_field`` to :class:`~django.db.models.DateTimeField`.
+
+.. versionchanged:: 3.2
+
+  Support for inferring a default ``output_field`` from the type of ``value``
+  was added.
 
 ``ExpressionWrapper()`` expressions
 -----------------------------------

docs/releases/3.2.txt

@@ -233,6 +233,15 @@ Models
 * The ``of`` argument of :meth:`.QuerySet.select_for_update()` is now allowed
   on MySQL 8.0.1+.
 
+* :class:`Value() <django.db.models.Value>` expression now
+  automatically resolves its ``output_field`` to the appropriate
+  :class:`Field <django.db.models.Field>` subclass based on the type of
+  it's provided ``value`` for :py:class:`bool`, :py:class:`bytes`,
+  :py:class:`float`, :py:class:`int`, :py:class:`str`,
+  :py:class:`datetime.date`, :py:class:`datetime.datetime`,
+  :py:class:`datetime.time`, :py:class:`datetime.timedelta`,
+  :py:class:`decimal.Decimal`, and :py:class:`uuid.UUID` instances.
+
 Requests and Responses
 ~~~~~~~~~~~~~~~~~~~~~~
 

tests/aggregation/tests.py

@@ -848,10 +848,6 @@ class AggregateTestCase(TestCase):
         book = Book.objects.annotate(val=Max(2, output_field=IntegerField())).first()
         self.assertEqual(book.val, 2)
 
-    def test_missing_output_field_raises_error(self):
-        with self.assertRaisesMessage(FieldError, 'Cannot resolve expression type, unknown output_field'):
-            Book.objects.annotate(val=Max(2)).first()
-
     def test_annotation_expressions(self):
         authors = Author.objects.annotate(combined_ages=Sum(F('age') + F('friends__age'))).order_by('name')
         authors2 = Author.objects.annotate(combined_ages=Sum('age') + Sum('friends__age')).order_by('name')
@@ -893,7 +889,7 @@ class AggregateTestCase(TestCase):
 
     def test_combine_different_types(self):
         msg = (
-            'Expression contains mixed types: FloatField, IntegerField. '
+            'Expression contains mixed types: FloatField, DecimalField. '
             'You must set output_field.'
         )
         qs = Book.objects.annotate(sums=Sum('rating') + Sum('pages') + Sum('price'))

tests/aggregation_regress/tests.py

@@ -388,7 +388,7 @@ class AggregationTests(TestCase):
         )
 
     def test_annotated_conditional_aggregate(self):
-        annotated_qs = Book.objects.annotate(discount_price=F('price') * 0.75)
+        annotated_qs = Book.objects.annotate(discount_price=F('price') * Decimal('0.75'))
         self.assertAlmostEqual(
             annotated_qs.aggregate(test=Avg(Case(
                 When(pages__lt=400, then='discount_price'),

tests/expressions/tests.py

@@ -3,15 +3,17 @@ import pickle
 import unittest
 import uuid
 from copy import deepcopy
+from decimal import Decimal
 from unittest import mock
 
 from django.core.exceptions import FieldError
 from django.db import DatabaseError, NotSupportedError, connection
 from django.db.models import (
-    Avg, BooleanField, Case, CharField, Count, DateField, DateTimeField,
+    Avg, BinaryField, BooleanField, Case, CharField, Count, DateField,
-    DurationField, Exists, Expression, ExpressionList, ExpressionWrapper, F,
+    DateTimeField, DecimalField, DurationField, Exists, Expression,
-    Func, IntegerField, Max, Min, Model, OrderBy, OuterRef, Q, StdDev,
+    ExpressionList, ExpressionWrapper, F, FloatField, Func, IntegerField, Max,
-    Subquery, Sum, TimeField, UUIDField, Value, Variance, When,
+    Min, Model, OrderBy, OuterRef, Q, StdDev, Subquery, Sum, TimeField,
+    UUIDField, Value, Variance, When,
 )
 from django.db.models.expressions import Col, Combinable, Random, RawSQL, Ref
 from django.db.models.functions import (
@@ -1711,6 +1713,30 @@ class ValueTests(TestCase):
         value = Value('foo', output_field=CharField())
         self.assertEqual(value.as_sql(compiler, connection), ('%s', ['foo']))
 
+    def test_resolve_output_field(self):
+        value_types = [
+            ('str', CharField),
+            (True, BooleanField),
+            (42, IntegerField),
+            (3.14, FloatField),
+            (datetime.date(2019, 5, 15), DateField),
+            (datetime.datetime(2019, 5, 15), DateTimeField),
+            (datetime.time(3, 16), TimeField),
+            (datetime.timedelta(1), DurationField),
+            (Decimal('3.14'), DecimalField),
+            (b'', BinaryField),
+            (uuid.uuid4(), UUIDField),
+        ]
+        for value, ouput_field_type in value_types:
+            with self.subTest(type=type(value)):
+                expr = Value(value)
+                self.assertIsInstance(expr.output_field, ouput_field_type)
+
+    def test_resolve_output_field_failure(self):
+        msg = 'Cannot resolve expression type, unknown output_field'
+        with self.assertRaisesMessage(FieldError, msg):
+            Value(object()).output_field
+
 
 class FieldTransformTests(TestCase):
 
@@ -1848,7 +1874,9 @@ class ExpressionWrapperTests(SimpleTestCase):
         self.assertEqual(expr.get_group_by_cols(alias=None), [])
 
     def test_non_empty_group_by(self):
-        expr = ExpressionWrapper(Lower(Value('f')), output_field=IntegerField())
+        value = Value('f')
+        value.output_field = None
+        expr = ExpressionWrapper(Lower(value), output_field=IntegerField())
         group_by_cols = expr.get_group_by_cols(alias=None)
         self.assertEqual(group_by_cols, [expr.expression])
         self.assertEqual(group_by_cols[0].output_field, expr.output_field)

tests/ordering/tests.py

@@ -1,7 +1,6 @@
 from datetime import datetime
 from operator import attrgetter
 
-from django.core.exceptions import FieldError
 from django.db.models import (
     CharField, DateTimeField, F, Max, OuterRef, Subquery, Value,
 )
@@ -439,17 +438,6 @@ class OrderingTests(TestCase):
         qs = Article.objects.order_by(Value('1', output_field=CharField()), '-headline')
         self.assertSequenceEqual(qs, [self.a4, self.a3, self.a2, self.a1])
 
-    def test_order_by_constant_value_without_output_field(self):
-        msg = 'Cannot resolve expression type, unknown output_field'
-        qs = Article.objects.annotate(constant=Value('1')).order_by('constant')
-        for ordered_qs in (
-            qs,
-            qs.values('headline'),
-            Article.objects.order_by(Value('1')),
-        ):
-            with self.subTest(ordered_qs=ordered_qs), self.assertRaisesMessage(FieldError, msg):
-                ordered_qs.first()
-
     def test_related_ordering_duplicate_table_reference(self):
         """
         An ordering referencing a model with an ordering referencing a model